The present invention relates to heat-shrinkable polyester films, and more particularly, it relates to heat-shrinkable polyester films having excellent low temperature shrinkability, suitable for applications such as labels and cap seals, which cause only rare occurrence of shrinkage spots, wrinkles, strains, folding and other defects after heat shrinkage of the film in the step of shrinking with hot air, which exhibit a quite small lowering in shrinkage finish after a lapse of time for storage, and further which have low spontaneous shrinkability.
In the past, heat-shrinkable films have widely been used for various applications, such as shrink-wrap films, shrinkable labels and cap seals, by making use of their property of causing shrinkage by heating. In particular, stretched films made of vinyl chloride resins, polystyrene resins or polyester resins have been used for labels on various vessels such as polyethylene terephthalate (PET) vessels, polyethylene vessels, and glass vessels.
However, vinyl chloride resins have serious problems including evolution of harmful gasses such as hydrogen chloride gas in their incineration. In addition, heat-shrinkable films made of vinyl chloride resins are used for shrinkable labels on PET or other vessels, the labels should be separated from the vessels and discarded in the recycling of vessels. Further, in the applications such as full labels, which require shrinkable labels to have high heat shrinkability depending on the shapes of vessels, heat-shrinkable films made of vinyl chloride resins have serious problems that they cannot be used because of their low heat shrinkability leading to the failure of shrinkage.
In contrast, films of polystyrene resins or polyester resins cause no evolution of harmful substances such as hydrogen chloride gas in their incineration, and therefore, these films have been expected to take the place of vinyl chloride rein films as shrinkable labels on vessels.
However, polystyrene resin films, although they exhibit good shrinkage finish in appearance after shrunk, have poor solvent resistance, so that they require the use of special ink in their printing. They also have serious problems in their disposal, e.g., they require incineration at high temperatures, in which case they may cause evolution of black smoke and bad smell in large quantities. They further have serious problems that they cannot be used in the applications which require shrinkable films to have high heat shrinkability because of their failure of shrinkage similarly to vinyl chloride resin films. For materials capable of solving these problems, polyester resin films have extremely been expected to serve, and their use has been increasing.
Even the above conventional heat-shrinkable polyester films, however, have not yet been quite satisfactory in their heat-shrinkage characteristics. In particular, they have serious problems that shrinkage spots, wrinkles, strains and folding easily occur during shrinkage in the step of shrinking with hot air; and when used to cover vessels such as PET bottles, polyethylene bottles and glass bottles, and then shrunk, letters or patterns previously printed on the films before shrinkage are distorted; and adhesion of the films to the vessels is insufficient. In addition, when stored at room temperature for a long period of time, they cause a remarkable lowering in shrinkability at low temperatures, so that shrinkage rapidly occurs making it further easy to form defects such as shrinkage spots, wrinkles and folding. The shrinkability at low temperatures raised too much to solve these problems leads to another serious problem that spontaneous shrinkability becomes higher.
Under the actual circumstances of the prior art, the present inventors have extensively studied heat-shrinkable polyester films, and as a result, they have reached the present invention. That is, to attain the above purpose, the heat-shrinkable polyester films of the present invention are characterized in that the heat shrinkability in the main shrinkage direction of the film after treatment in hot water at 80xc2x0 C. for 10 seconds is 40% or higher and the change of heat shrinkability in the main shrinkage direction of the film after treatment in hot water at 55-75xc2x0 C. for 10 seconds before and after the film is left under an atmosphere at 40xc2x0 C. for 160 hours is 15% or lower.